The Casady High School student disappeared April 11, 2014.
Prosecutors said they believe she was murdered by two Edmond teenagers, Chadd Raymond, 17, and Chloe Thomas, 17.
As part of an agreement with the state, Raymond pleaded guilty to his role in the murder, Friday afternoon.
Private investigator Darcie Parton-Scoon spoke on behalf of the family, "[We are all] certainly pleased about the plea agreement. I'm pleased that the family doesn't have to go through that long trial," she said.
Raymond will serve 35 years of a life sentence in exchange for his testimony against Chloe Thomas.
Thomas implicated her co-defendant, Raymond earlier this year.
In her confession, Thomas allegedly told investigators Raymond strangled Hill at his brother's apartment in Edmond.
Investigators said Thomas claimed she only assisted Raymond after the killing, when the two allegedly loaded Hill's body into her car, and drove to an unknown heavily wooded area to dispose of her body.
Hill's body has never been recovered. As part of the plea deal, Raymond must go out with investigators to help them find Hill's body.
Prosecutors are now convinced Thomas made up that story to frame Raymond, according to the murder charge filed against Thomas, Friday,
She was expected to plead guilty to a lesser charge in juvenile court later this month.
Now, the tables have turned.
Raymond recently gave investigators a confession that contradicts Thomas' earlier statements.
New court documents showed Thomas actually devised a plan to rob Hill and take her car.
Raymond told investigators he panicked when the plan went awry and decided he had to kill her.
Raymond reportedly choked Hill until he became too tired and that's when Thomas allegedly took over.
The affidavit showed Thomas choked Hill until she quit breathing.
"We had a plea deal," Michael Trevino, Thomas' attorney said. He also said he was surprised by the turn of events because she has been cooperating with the district attorney since the beginning.
Trevino also said, "I find it odd that the district attorney struck a deal with the person who killed her to testify against the person who turned him in."
"It is hard for me to give a more in-depth comment because I still have not been contacted by the district attorney office, even though they know I represent her," Trevino said.
Prosecutors did not give comment because the case is pending.